This invention relates to a receiver element for use in thermal mass transfer imaging applications and, more particularly, to such a receiver element which includes a nanoporous, ultrasmooth image-receiving layer. The invention also relates to a thermal transfer imaging system including the receiver element.
A number of different printing systems make use of thermally induced transfer of a colorant, such as a dye, from a donor element to a receiver element. In some of these systems the dye alone diffuses from a polymeric binder on the donor element to another polymeric layer on the receiver element, whereas in others, a vehicle (which may be a polymeric binder, a wax, or a combination of the two) and the dye are transferred together from the donor element to the receiver element. The latter process is commonly referred to as thermal mass transfer.
There are known in the art a number of different types of donor elements for use in thermal mass transfer imaging. For example, waxes or resins are commonly reported as vehicles or binders, while dyes or pigments may be used as colorants.
There are also known in the art various types of receiving elements for use in thermal mass transfer imaging. Certain of these receiving elements contain materials which soften at imaging temperatures in order to absorb transferred materials. Such a receiver element, for example, is described in U.S. Pat. No. 4,686,549. However, an alternative, and often preferable, receiver element uses receiving materials which are surface porous, so that the heated donor material adheres preferentially to the receiver by fully or partially flowing into the pores of the receiver element. For example, U.S. Pat. Nos. 5,521,626 and 5,897,254 describe the transfer of material from a heated donor element to a surface-porous receiver sheet in which the pore diameter is in the range of 1-10 micrometers. U.S. Pat. No. 5,563,347 describes a similar system. Unfortunately, in these prior art examples, the size of the pores in the receiver sheet is sufficient to scatter visible light, and as a result the receiver element has a matte appearance.
Surface porous receiver coatings have been devised for ink jet printing in which the average pore diameter is considerably less than one micrometer (usually in the range of about 0.02-0.2 xcexcm). Such surface porous layers are herein referred to as nanoporous. Pores of this small size do not appreciably scatter visible light, and therefore the receiver sheet can have a glossy appearance. For example, receiver sheet compositions described in U.S. Pat. Nos. 5,612,281 and 6,165,606, directed for use in inkjet printing, have the characteristics of being nanoporous and glossy. The viscosity of a typical ink is considerably lower than that of the conventional thermal mass transfer materials described above (at their transfer temperature), and consequently ink can penetrate the smaller pores of the nanoporous receiver coatings whereas the molten conventional mass transfer donor materials cannot.
There are, however, properties required of a receiver element for thermal mass transfer which these prior art ink-jet receiver elements do not possess. Some of these additional required properties result from the method by which a thermal mass transfer process is used to produce images approaching photographic quality. The resolution of an image produced by a thermal transfer process employing a page-wide array of heating elements (commonly referred to as a xe2x80x9cthermal print headxe2x80x9d) is limited by the resolution of the thermal print head employed. In a typical printing arrangement, the donor and receiver elements are brought together, and the resulting laminar assembly is translated beneath the thermal print head. Electrical current is supplied only to those heating elements corresponding to pixels which are to be colored in the line of the image being printed at a particular time. Thus, a thermal print head having, say, three hundred heating elements per inch can transfer only three hundred dots per inch from the donor element to the receiver element in the direction transverse to the motion of the two elements relative to the print head. (Obviously, more than three hundred dots per inch may be printed in the direction of motion). If the transferred dots are all equal in size, each pixel in the final image will only have two possible levels of gray: either full dye density (Dmax) or no dye density (Dmin). At a (typical) resolution of three hundred dots per inch, this number of gray levels is insufficient to produce an image of photographic quality. In some prior art thermal mass transfer imaging processes, as described for example in xe2x80x9cA New Thermal Transfer Ink Sheet for Continuous-Tone Full Color Printerxe2x80x9d, by M. Kutami, M. Shimura, S. Suzuki and K. Yamagishi, J. Imaging Sci., 1990, 16, 70-74, the attempt is made to attain the numerous shades of gray necessary to produce an image of photographic appearance by changing the size of a dot (of constant dye density) within the limitation on pixel spacing imposed by the resolution of the thermal print head.
One confounding factor in producing images of high quality by means of dot size variation is the problem of graininess. Graininess is caused by lack of precise control in the size of dots printed. Whereas a field of identical small dots will appear to the eye to have a smooth appearance (provided that the individual dots cannot be resolved), a field of dots of the same average size, but with a broader distribution of sizes around the average, may acquire a grainy, or mottled, appearance.
If the receiving element in a thermal mass transfer imaging process is not sufficiently flat and smooth, the contact between the donor element and the receiving element may be uneven. Such uneven contact may lead to the formation of dots of uncontrolled size (since transfer will be more efficient onto xe2x80x9chillsxe2x80x9d than into xe2x80x9cvalleysxe2x80x9d), and this will be manifested as a grainy appearance to the image. The prior art ink jet receiving elements described above typically do not have the flatness and smoothness required to avoid unacceptable graininess when used in a thermal mass transfer process in conjunction with a thermal print head.
Other desirable properties for thermal transfer receiver elements have also been described in the prior art. In order to ensure an even contact between the donor and receiver elements across the whole width of a thermal head during printing, some compressibility of the receiver element is preferred. In addition, in order that the heat provided by the thermal print head be used as efficiently as possible, the receiver element preferably has a low thermal conductivity. Thus, for example, U.S. Pat. No. 5,244,861 describes a receiving element comprising a substrate having a dye image-receiving layer. The substrate is a composite film made up of a microvoided thermoplastic core layer and at least one substantially void-free thermoplastic surface layer. The microvoided thermoplastic core provides the necessary compressibility and low thermal conductivity for the receiver element. The thermal conductivity of the receiving element should also be spatially uniform in directions parallel to the image plane. Non-uniformities in thermal conductivity will be manifest as dye density variations in an image produced by a thermal transfer technique. This is because the temperatures to which the donor and receiver elements are heated by a given heating pulse from the thermal print head depends upon the rate of loss of heat by conduction through the receiver substrate, and the dye density achieved is a function of these temperatures.
As the state of the thermal imaging art advances efforts continue to be made to provide new thermal imaging systems that can meet new performance requirements, and to reduce or eliminate some of the undesirable characteristics of the known systems. It would be advantageous to have a receiver element for use in thermal mass transfer imaging applications that can provide images having a glossy appearance.
It is therefore an object of this invention to provide a novel receiving element for use in thermal mass transfer imaging applications.
It is another object of the invention to provide a receiver element that has a nanoporous, ultrasmooth receiver layer.
It is still another object of the invention to provide a receiver element having a glossy appearance.
Yet another object of the invention is to provide a receiver element which can minimize visible image defects caused by small particles entrapped between the donor and receiver elements during formation of the image;
It is a further object of the invention to provide a receiver element that can provide stabilization for images deposited on the receiving element, both with respect to lateral diffusion of transferred colorants and to photofading of transferred colorants.
Still further, it is an object of the invention to provide a thermal mass transfer imaging system.
These and other objects and advantages are accomplished in accordance with the invention by providing a receiving element for use in mass transfer thermal imaging applications which comprises a substrate carrying on a surface thereof, a nanoporous, ultrasmooth receiver layer. The substrate may comprise a material having a compressibility of at least 1% under a pressure of 1 Newton per square millimeter (1 MPa). Optionally, there may be provided, between the substrate and the nanoporous receiving layer, a layer having a thickness of less than about 50 xcexcm which is comprised entirely of a material having a compressibility of less than about 1% under a pressure 1 MPa. Alternatively, the substrate may comprise only the material having a compressibility of less than about 1% under a pressure of 1 MPa, provided that the thickness of the substrate does not exceed about 50 xcexcm.
The image-receiving layer has a uniformly voided structure having a porous surface. The void volume of the image-receiving layer is between about 40% and about 70%. The pore diameter distribution, sampled at regularly spaced intervals, is such that at least about 50% of pores having diameters larger than about 30 nm have diameters smaller than about 300 nm, and at least about 95% of pores having diameters larger than about 30 nm have diameters smaller than about 1000 nm. In a particularly preferred embodiment, at least about 50% of pores having diameters larger than about 30 nm have diameters smaller than about 200 nm, and at least about 95% of pores having diameters larger than about 30 nm have diameters smaller than about about 500 nm.
It is preferred that the root mean square (RMS) surface roughness of the image-receiving layer, as measured over an area of about 1.5 mm by about 1.5 mm, be less than about 300 nm. A particularly preferred RMS surface roughness is less than about 200 nm.
The image-receiving layer comprises a particulate material that may be organic or inorganic and a binder. In a preferred embodiment the binder is a hydrophobic polymeric material. In another preferred embodiment the receiver element further comprises a wash coating which is deposited on the image-receiving layer in such a manner that additional addenda such as photostabilizers are introduced into the nanoporous receiving layer.
There is also provided a thermal mass transfer imaging system which comprises the advantageous receiver element and a donor element having a solid thermal transfer material which is converted to a low viscosity fluid at the thermal imaging temperature.